Testing the reliability of quartz OSL ages beyond the Eemian

Radiation Measurements 43 (2008) 776–780www.elsevier.com/locate/radmeas

Testing the reliability of quartz OSL ages beyond the Eemian

Andrew Murraya,∗, Jan-Pieter Buylaerta, Mona Henriksenb,John-Inge Svendsenb,c, Jan Mangerudb,c

aNordic Laboratory for Luminescence Dating, Department of Earth Sciences, Aarhus University, Ris�---DTU, DK-4000 Roskilde, DenmarkbDepartment of Earth Science, University of Bergen, Allégaten 41, N-5007 Bergen, Norway

cBjerknes Centre for Climate Research, University of Bergen, Allégaten 55, N-5007 Bergen, Norway

Abstract

There is some evidence that optically stimulated luminescence (OSL) dating using the fast component from quartz may underestimate theage by ∼10% at about 130 ka, or for values of De of ∼150.200 Gy. Any significant underestimate in age, if true, would be inconsistent withthe expected values for the stability of the OSL trap. However, the alternative explanation, that the age control provided by the beginning ofmarine isotope stage 5e is ∼10% too old, would be very contentious. As part of our continuing investigation of the reliability of quartz agesfor older material, we describe results from a deposit on the Seyda River in northern Russia. Quartz was extracted from an organic-rich layeridentified as being laid down in either MIS 7.1 (∼193 ka) or MIS 7.3 (∼215 ka), and U-series dated to 198 ± 7 ka, and from the immediatelyoverlying fluvial sand. The resulting mean OSL age of 10 samples from these sedimentary units is 207 ± 12 ka. The results are also discussedin relation to the earlier quartz studies.© 2008 Elsevier Ltd. All rights reserved.

Keywords: OSL dating; Eemian; Quartz; Underestimation

1. Introduction

It is known that optically stimulated luminescence (OSL)ages based on the fast component from quartz appear to beless reliable as the growth curve approaches saturation. Variousauthors have produced evidence that these OSL signals mayunderestimate the age by ∼10% at about 130 ka, or for valuesof De of ∼150.200 Gy (e.g. Murray and Funder, 2003; Stokeset al., 2003; Murray et al., 2007). On the other hand, Watanukiet al. (2005) found good agreement with independent age con-trol at ∼350 ka, and acceptable agreement at 600 ka, althoughthese results were very dependent on water content assump-tions. Various mechanisms have been put forward to explaininaccuracies in quartz ages at high doses, including changes intrap and luminescence centre competition, and trap instability.Based on model predictions, Bailey (2004) and Bailey et al.(2004) suggested that relative changes in trapped charge popu-lations in thermally stable and unstable traps and centres should

∗ Corresponding author. Tel/fax.: +45 46 77 59 79.E-mail address: andrew.murray@risoe.dk (A. Murray).

1350-4487/$ - see front matter © 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.radmeas.2008.01.014

give rise to differences between luminescence response to agiven dose in nature compared to that observed during labo-ratory measurements. They produced some experimental ev-idence to support their prediction. However, this mechanismshould lead to an overestimate of age, rather than the underes-timate observed. Wintle and Murray (2006) summarise the lab-oratory evidence for the thermodynamic stability of the OSLtrap; this evidence indicates a lifetime at ambient temperatureof ∼108 years and such a value would be inconsistent with ageunderestimates at only ∼105 years. However, the alternativeexplanation, that the age control provided by the beginning ofmarine isotope (MIS) stage 5e is ∼10% too old, would be verycontentious.

In this paper, we examine the accuracy of the OSL ages de-rived from quartz extracted from a deposit on the Seyda Riverin the Pechora Lowland, northeastern European Russia. A 50 mhigh bluff cut by the river reveals two till beds separated by aninterglacial unit consisting of organic-rich material overlain byfluvial sand and silt, and compacted by later glaciation (Fig. 1).Both pollen analysis and U-series dating constrain the age ofthe organic material to ∼200 ka, and so provide an independent

A. Murray et al. / Radiation Measurements 43 (2008) 776–780 777

Fig. 1. Schematic diagram of the sampled section with sampling positions, SAR OSL ages and U-series dates indicated. (The uncertainties on the U-seriesages have been simplified from Table 1 for ease of presentation.)

age control for the quartz ages. We first present a summary ofthe luminescence characteristics of quartz extracted from theSeyda deposits. We then summarise the OSL dating results,and compare these with the U-series ages. Finally, we discussthis comparison in the light of earlier published comparisonsbetween OSL and other independent age control in this agerange.

2. Sampling, age control and measurement facilities

The Seyda River site (67◦23′N, 62◦55.8′E) lies in thePechora Lowland, north-eastern European Russia; a full sitedescription is in preparation (Henriksen et al., in preparation)In brief, a 50 m high bluff (Fig. 1) cut by the river reveals twotill beds (units A and C) separated by an interglacial unit B.This interglacial unit consists of organic-rich material over-lain by fluvial sand and silt, and has been compacted by laterglaciation. The overlying unit C consists of clayey till withsome sand lenses and this, in turn, is overlain by glaciofluvialgravel and sand (unit D). Within the incised river valley, afluvial and an aeolian sand (unit E) then accumulated.

Ten luminescence samples were collected from within andimmediately overlying the ∼1 m thick peat and sand layer, to-gether with eight other samples to give confidence in the over-all stratigraphic interpretation, by driving opaque plastic tubes(∼8 cm diameter, 40 cm long) into a cleaned face. The varioussedimentary units and the relative locations of the samples areindicated diagrammatically in Fig. 1.

Age control is provided by the U/Th ages of three samples(field sample codes 98-3015, 98-3016 and 98-3018) taken fromwithin the peat-rich layer, the first two of which were analysedtwice by different techniques (Table 1, see also Astakhov,2004). Three other samples were taken from this unit, butthey were rejected by the analytical laboratory on the groundsof open-system behaviour; unusually high U/Th parent ratiosin these samples are thought to indicate continued uranium

Table 1U-series ages

Lab. sample Field sample Age (ka)

4315a 98-3015 209 + 19/ − 144315b 98-3015 201 + 16/ − 144316a 98-3016 181 + 13/ − 114316b 98-3016 194 + 21/ − 174319 98-3018 236 + 26/ − 21

absorption long after deposition. (All U/Th analyses and cal-culations were undertaken by V. Kuznetsov of the Laboratoryof Geochronology, St. Petersburg University, Russia). Theweighted mean of the five accepted analyses (final column inTable 1) is 198 ± 7 ka.

Further age control can be derived from examination of thepollen ensemble in the peat layer. According to Finnekås (1998)this is indicative of an environment warmer than the present.Given the range of the U/Th ages, and of the OSL ages pre-sented later, there are only two possible candidates from theMIS record that meet this condition: either MIS 7.1 (∼193 ka)or MIS 7.3 (∼215 ka) (Martinson et al., 1987). Thus, we areconfident that the interglacial organic-rich material and overly-ing fluvial sand and silt were deposited around 193 or 215 ka,and take the weighted mean of the U/Th ages, of 197 ± 8 ka,as the best estimate.

Quartz-rich extracts (180.250 �m) were separated in theusual manner (sieving and acid treatment, but without theuse of heavy liquids), and tested for the absence of feldsparcontamination using infrared stimulation. Measurements wereperformed using a standard RisZ TL/OSL reader with bluelight (470 nm; ∼50 mW cm−2) stimulation, OSL detection wasthrough 7 mm of U-340 filter, and a calibrated beta source de-livering ∼0.15 Gy s−1. A SAR protocol (Murray and Wintle,2000) was used throughout (unless otherwise indicated, 260 ◦Cpreheat for 10 s and 220 ◦C cut-heat), together with a high

778 A. Murray et al. / Radiation Measurements 43 (2008) 776–780

temperature stimulation for 40 s at 280 ◦C at the end of eachSAR cycle (Murray and Wintle, 2003). The initial 0.32 s ofthe OSL signal less a background integrated between 3.2 and3.9 s was used for all dose calculations. Dosimetry is based onhigh resolution gamma spectrometry (Murray et al., 1987) andlaboratory beta counting, with water contents derived from lab-oratory measurements of saturated samples (Table 2). Becauseof the sampling depth and local climate, it is assumed thatthe sediment was saturated with water throughout the burial

5L/T=2.66(1-e-D/67)+D/360

4 Natural

2

3

De=398 Gy

Stimulation time, s0

15x103

1

5x103

10x103

Natural

Laboratory dose, Gy0

0200 400 600 800

Cor

rect

ed lu

min

esce

nce,

Lx/T

x

10 20 30

Fig. 2. Typical SAR growth curve of a single aliquot from sample 992502,showing repeat points as open symbols. The intersection of the naturalluminescence gives the De for this aliquot. A typical natural OSL decaycurve is shown inset.

Table 2Summary of dosimetry, De measurements and luminescence ages

Samplecode

Burial depth(cm)

226Ra(Bq kg−1)

232Th(Bq kg−1)

40K(Bq kg−1)

Total dose rate(Gy kg−1)

wc(%)

De

(Gy)(n) Age

(ka)

992529 120 15.6 ± 0.3 17.9 ± 0.3 458 ± 9 1.81 ± 0.07 27 209 ± 7 6 115 ± 6992528 760 5.7 ± 0.3 6.1 ± 0.3 242 ± 9 0.90 ± 0.04 27 151 ± 6 6 167 ± 11002539 170 10.4 ± 0.3 9.9 ± 0.3 384 ± 9 1.44 ± 0.06 27 230 ± 8 6 160 ± 9002540 310 12.4 ± 0.6 14 ± 0.5 398 ± 16 1.52 ± 0.07 27 250 ± 10 6 164 ± 10002541 370 16.6 ± 0.4 18.9 ± 0.4 353 ± 9 1.52 ± 0.06 27 277 ± 13 6 182 ± 12002542 240 13.7 ± 0.4 15.5 ± 0.4 521 ± 13 1.87 ± 0.07 27 298 ± 19 6 160 ± 13992512 1700 13.9 ± 0.2 15.5 ± 0.2 547 ± 10 1.82 ± 0.07 27 421 ± 15 6 231 ± 13992513 1700 12.3 ± 0.5 14.6 ± 0.4 520 ± 16 1.73 ± 0.08 27 369 ± 11 6 213 ± 12992515 2200 17.1 ± 0.4 18.6 ± 0.4 461 ± 11 1.69 ± 0.07 27 356 ± 41 6 210 ± 26962523 2200 # # # 1.57 ± 0.08 27 360 ± 16 18 229 ± 16992511 2200 12.8 ± 0.3 14.4 ± 0.3 383 ± 8 1.36 ± 0.06 27 258 ± 28 5 189 ± 22962525 2300 # # # 1.21 ± 0.06 27 301 ± 12 18 250 ± 18032512 2200 30.1 ± 0.5 28.1 ± 0.5 461 ± 9 1.58 ± 0.06 46 263 ± 29 5 166 ± 20992502 2200 25.4 ± 0.5 22.2 ± 0.5 433 ± 11 1.78 ± 0.08 27 401 ± 29 6 226 ± 19962524 2300 # # # 1.43 ± 0.07 27 308 ± 14 18 216 ± 15992501 2300 16.8 ± 0.6 12.6 ± 0.5 340 ± 14 1.31 ± 0.06 27 285 ± 34 6 218 ± 28032509 2300 44.1 ± 1.0 30.3 ± 1.0 493 ± 15 2.06 ± 0.10 23 387 ± 21 6 188 ± 14032510 2300 42.7 ± 0.8 28.0 ± 0.7 492 ± 12 2.27 ± 0.10 23 405 ± 44 6 178 ± 21

Note: 1. Dry dose rates derived from concentration measurements using data given in Olley et al. (1996).2. (n) is the number of aliquots measured to give the average De.3. # denotes radionuclide concentrations not measured. Dose rates derived from laboratory beta counting.4. Total dose rates include water content correction and cosmic ray contribution. Saturated water contents (wc, %) assumed to apply throughout burial period.Cosmic ray dose rates derived from Prescott and Hutton (1994).5. All uncertainties include estimates of both random and systematic contributions in a 67% confidence interval.

period. No special dosimetry assumptions were made for theorganic-rich layer (in unit B). It was sufficiently thick to allowthe assumption that all the gamma dose rate originated withinthe layer, and any compression and dewatering can be reason-ably assumed to have happened shortly after deposition, whenthe unit was over-run by the immediately following glaciation.Thus, the assumption that the saturation water content mea-sured today has applied throughout the site lifetime is also con-sidered to apply to this organic layer.

3. Luminescence characteristics

In common with most material from northern Russia andSiberia, the quartz OSL is dominated by a strong fast compo-nent (Fig. 2, inset). The SAR laboratory growth curve for thismaterial can be well represented by the sum of a single saturat-ing exponential and a linear component (Fig. 2); the recyclingratio is close to one (repeat points shown as open symbols)and the growth curve passes very close to the origin. For thetypical dose rate at this site (∼1.8 Gy ka−1; see Table 2) anage of 200 ka corresponds to a natural dose of ∼350 Gy. Suchnatural doses lie well onto the high dose linear region of thegrowth curve, considerably above the twice D0 limit suggestedby Wintle and Murray (2006) as being suitable for dating.

Fig. 3 shows a typical preheat plateau and associated recy-cling ratios and recuperation. All the estimates of De in theupper part of the figure are indistinguishable from the meanof 335 Gy, although there may be a tendency for a systemati-cally lower value at lower preheat temperatures. This may beassociated with the corresponding tendency for the recycling

A. Murray et al. / Radiation Measurements 43 (2008) 776–780 779

600

800 992511

De,

Gy

200

400De (160-280 °C) = 335 Gy

1.44

50

0.8

1.0

1.2

2

3mean (160-280 °C) = 1.031 ± 0.009

160

Rec

yclin

g ra

tio

0.6

Rec

uper

atio

n, %

Nat

ural

0

1

Preheat temperature,°C 180 200 220 240 260 280 300

Fig. 3. Dependence of De, recycling ratio and recuperation on preheat tem-perature for sample 992511. Three aliquots were measured per preheat tem-perature; error bars represent one standard error.

20

Mean 1.03±0.02RSD=15%n=48

Freq

uenc

y

10

15

0

5

Measured/Given Dose0.4 0.6 0.8 1.0 1.2 1.4 1.6

Fig. 4. Histogram summarising all the dose recovery results.

ratios to lie above unity at lower temperatures (lower figure),and should not necessarily be taken to indicate a contributionfrom unstable low temperature traps. As would be expectedfor such older material, the recuperated signal is negligiblecompared to the size of the natural throughout the entire preheattemperature range.

Three natural aliquots of each of the 16 samples were thenbleached at room temperature using blue light (2 times 250 swith a 10 ks pause in between), and given a dose of ∼300 Gy

before any preheating. This dose was then measured in theusual manner, and a histogram of the 48 measured to given doseratios is shown in Fig. 4. The mean ratio is 1.03±0.02 (n=48)

indicating that we are able to accurately measure such a highdose given before any thermal treatment of the sample. Thisgives us confidence that our laboratory measurement procedureis reliable, but it does not necessarily guarantee that we canaccurately measure a natural dose of the same order.

4. Results

Fig. 1 presents a summary of the OSL ages for this section.The first observation is that they are stratigraphically consis-tent, with the sediments from units D and E being consistentlyyounger than the units in the underlying section. The glacio-tectonised sand lens (presumably transported as a clast) con-tained within the till unit C is also indistinguishable in agewith the underlying interglacial peat and fluvial sand, as wouldbe expected. Finally, the samples from within the peat andthe overlying fluvial sand (unit B) all give very similar ages.Fig. 5 presents the observed De values plotted against the doserates (see Table 2) for the samples from unit B to which theage control applies. Despite being derived from well up thedose–response curve, and involving a range in dose rates ofmore than a factor of two, they are all consistent with the solidline of slope 197 ka, based on the U-series results. If system-atic errors were to arise at higher doses, these data would notbe consistent with a line passing through the origin; this goodcorrelation over a wide range of De values gives us consider-able confidence that such systematic dose dependent effects arenot significant.

The mean OSL age for the samples taken within the organic-rich layer is 200 ± 10 ka (n = 4) and that for the samples fromthe immediately overlying fluvial layer is 212 ± 12 ka (n = 6).Not surprisingly, these two beds cannot be distinguished inage, and the overall weighted mean is 207 ± 12 ka (n = 10).

500

400

200

300

Equ

ival

ent d

ose,

Gy

100Predicted slope basedon mean U series age

0.00

Dose rate, Gy.ka-1

0.5 1.0 1.5 2.0 2.5

Fig. 5. Relationship between equivalent dose (De) and dose rate, showing thatall OSL ages are consistent with the straight of slope equal to the U-seriesage of 197 ka.

780 A. Murray et al. / Radiation Measurements 43 (2008) 776–780

Note that the uncertainties on the weighted mean are dominatedby systematic uncertainties, which is why they do not reducesignificantly when the average is taken. This mean OSL agecompares very favourably with the mean U-series age, of 197±8 ka and with the possible ages of ∼193 and ∼215 ka basedon orbital tuning; there is no suggestion of any discrepancybetween these three independent age estimates.

5. Discussion and conclusions

The excellent agreement of the OSL ages and the indepen-dent age controls in unit B (Fig. 1), taken together with thestratigraphic consistency of the OSL ages from overlying unitsD and E, is very encouraging. These results are in contrast tothose from the Eemian (125–130 ka) Gammelmark and Sulasites (Murray and Funder, 2003; Murray et al., 2007) for whicha small underestimate of between 10% and 15% was observed,and with other age comparisons between OSL and Eemiansamples (Stokes et al., 2003; Schokker et al., 2004). Further-more, Buylaert et al. (2008 a, b) have also dated the Gammel-mark and Sula sites using K-feldspar; in the first case feldspargave an age 5 ± 5% older than quartz, in the second, 1 ± 3%younger. Thus, it appears that feldspar and quartz give essen-tially indistinguishable ages at these two sites. (Wallinga et al.,2007 also used both phosphors, but the uncertainties on theiranalyses in the relevant age range are too large to be usefulin this context.) It should be noted that, over the time periodof interest here, all these studies relied on indirect age controlthrough faunal identification and/or oxygen isotope correlation.Our study compares results from two independent instrumentaldating methods applied directly to the sediments of interest.

At least for the samples from our study, the results obtainedfrom the high dose linear region of the growth curve appear tobe accurate. Based on these data, we conclude that there is noevidence for any unexpected instability of the quartz OSL trapgiving rise to the fast component; it thus seems unlikely thatthe age discrepancy reported earlier can be attributed to suchinstability.

Acknowledgements

Financial support from the Nordic Centre of Excellence pro-gramme of the Joint Committee of Nordic Natural ScienceResearch Councils is gratefully acknowledged. The technicalassistance of Mette Adrian, Anna-Birgit SZrensen and VickiHansen is very much appreciated. Ms. Saiko Sugisaki is thankedfor help with the De measurements.

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